Create and work on an ext4 filesystem image inside of macOS using Docker for Mac. Use this method if you aren't using external USB drives and/or don't want to install a fat VM like VirtualBox.

1. Install Docker

Follow the steps outlined on Docker's website.

2. Create a working folder

Pick a directory to start in: e.g. mkdir -p $HOME/ext4fs

3. Run an Ubuntu container

Run the container in privileged mode and mount the directory you just created:

docker run --privileged -it -v $HOME/ext4fs:/ext4fs ubuntu:latest bash

4. Create a raw disk image

Create a blank file and format it as ext4:

dd if=/dev/zero of=ext4fs.img bs=1G count=0 seek=6
# or set "seek=" to however many gigabytes you want
mkfs.ext4 ext4fs.img
losetup -fP --show ext4fs.img
# this will return a /dev/loop ID, e.g. /dev/loop0
mount /dev/loop0 /ext4fs

5. Verify the image is mounted

Enter the directory where the image is mounted to see if it formatted correctly:

root@b710398aa9e9:/# cd /ext4fs
root@b710398aa9e9:/ext4fs# ls -lah
total 20K
drwxr-xr-x 3 root root 4.0K Dec  5 23:47 .
drwxr-xr-x 5 root root  160 Dec  5 23:46 ..
drwx------ 2 root root  16K Dec  5 23:47 lost+found

6. Profit

When finished, you can unmount the directory (umount /ext4fs) and exit the container. Then on your host filesystem you'll have a shiny new raw ext4 image which you can then use other tools to convert to other image types, e.g. VirtualBox, VMWare, etc., using tools like Packer.

Note: this method does not mount external drives, although it may be possible to mount a partition from a USB drive (see this article). You can mount other external disk images, if you copy them into and mount another directory alongside your ext4 filesystem:

docker run --privileged -it --rm -v $HOME/ext4fs:/ext4fs -v $HOME/otherdir:/otherdir ubuntu:latest bash

You can then use any tools available to Ubuntu to work on other disk images or partitions while inside the container, having full read-write access to your ext4 partition.

Note: your mounted filesystem will not be visible to the host OS, as it's mounted in a loopback inside the container's OS.